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Other Terms: Radio
The radius is the lateral, slender, rod-like bone of the antebrachium. The rod-like shaft expands at both ends. The proximal end forms a wheel-like head with a proximal concavity, while the distal end expands medially to laterally to form the widest measure of the bone This end is concave anteriorly and convex and grooved posteriorly. Its ridge-like borders give it a triangular shape in cross section.
The word radius comes from the Latin meaning a stake or staff, a measuring-rod, spoke, or ray. Early anatomists fancied this rod-like bone's resemblance to the spoke of a wheel. Its first known use was by the Roman medical writer Celsus.
The radius articulates with four bones: the humerus, ulna, scaphoid, and lunate. The radius has three articular surfaces. The radial head articulates proximally with the capitulum of the humerus. Medially the radial head articulates with the radial notch of the ulna. Distally the carpal articular surface forms a joint with the scaphoid and lunate bones.
The radius ossifies from three separate centers, with a fourth center sometimes forming in the radial tuberosity. A diaphyseal center appears first. It forms in the center of the shaft during the eighth week of embryonic life and at birth has progressed to each end of the diaphysis. A second center forms a distal epiphysis during the first postnatal year, fusing with the diaphysis between the seventeenth and nineteenth year. The final center, the proximal epiphysis, forms in the fourth to fifth year and joins the diaphysis in the fourteenth to seventeenth year.